1. Field of the Invention
The present invention relates to digital image processing, and in particular to a method for generating high addressability binary images from gray video image data.
2. Description of Prior Art
Digital reproduction, transfer or display of original images on image output terminals begins by creating a digital representation of an original image. Commonly, the digital representation becomes a two-tone microstructure otherwise known as a bitmap. In such representations, multiple gray levels or gray densities in the original image are reproduced by varying the spatial frequency of halftone microstructures (or halftone cells/dots). Continuous tone images or image portions are typically represented in binary format by creating halftone cells or dots, where each cell represents a gray level density within an area of picture elements (pixels).
Methods of halftone digital image processing, encompassing the process steps of scanning or image acquisition through printing or display are known. In general, digital image output terminals (e.g. printers) are capable of creating spots within an area with a predetermined resolution (dots per unit area). in scanners, a single "spot" describes a multi-bit density response. Typically, input scanners may acquire density information at 256 levels of gray to describe a spot or pixel. By contrast, output terminals generally have two or another relatively small number of levels to reproduce image information. Output terminals commonly contend with excess scanned information by quantizing the image data through halftoning techniques,to represent the image as a halftone.
A continuing goal in the art is to develop output terminals such as printers with improved image quality. Physical constraints such as output terminal device resolution (spots per unit area) can be enhanced so as to increase perceived resolution without resorting to physical device resolution increases. Current technology improves device resolution without actually increasing dots per unit area. There are many advantages to increasing or controlling a printer's virtual (enhanced) resolution. The most evident advantage is a compatibility issue. Many printers have a physical resolution (or addressability) of 300 dots per inch (dpi), and by creating printers with greater physical resolutions such as 400 dpi, image emitters or document creators producing resolution dependent images at a particular resolution are no longer compatible. A solution to this problem is found by using device independent document (or image) descriptions such as Xerox Interpress and PostScript page description languages. Currently, however, other PDLs such as Adobe Hewlett Packard HP-PCL are not resolution independent.
A further limitation of higher resolution devices is hardware affordability. Physical memory requirements increases when device resolution increases, directly increasing the cost of a physical device. As a result of increased memory requirements for higher resolution devices, technology enhancing physical device resolution limitations (virtual resolution) are desirable additions to the physical device.
High addressability techniques use laser modulation or pulse width modulation to increase printer resolution without modifying the physical printer device. Laser modulation uses a controller to modulate the appearance of the final output image. Printed spots of the output image may be moved, shrunk, or reshaped to achieve a variety of effects. High addressability methods affect the horizontal resolution. For example, doubling printer modulation rate results in doubling the horizontal resolution, while keeping vertical resolution unchanged. New and improved techniques to increase image resolution have improved halftone image quality. Specifically in the field of digital halftoning different methods of converting continuous tone to binary .images while preserving the appearance of tonal gradation or density similar to the original image have been disclosed.
Kawamura U.S. Pat. No. 4,553,173 relates to an apparatus for displaying halftone images with a high resolution and a high gradation. Tai et al. U.S. Pat. No. 4,959,730 relates to apparatus which suppresses false density contours caused by an insufficient number of output gray levels in a reproduction system. Eschbach U.S. Pat. No. 5,045,952, describes improving image quality by dynamically adjusting the threshold of an error diffusion algorithm in accordance with the input image to selectively control the amount of edge enhancement introduced into a binary encoded output. Janeway U.S. Pat. No. 4,251,837, describes a method and apparatus for mode selection in a three decision thresholding mode switch to control copying of mixed format documents. Japanese Patent No. 58-139282 to Tanimoto describes a system to detect a picture position with high resolution by finding an intersection position between a threshold and a video signal from a strait interpolation corresponding to the counted result of a transfer clock between pixels putting adjacent thresholds between them.
Methods for converting a digital image signal into an analog image signal, where the analog image signal is compared to a periodic analog pattern signal such as a triangle wave signal to produce a pulse-width modulated image signal, includes Kobayashi U.S. Pat. No. 4,926,248 which describes a method for improving the image quality in an image recording apparatus, and for varying the recording area of each pixel in order to express light and shade of the image. Aral U.S. Pat. No. 4,847,695 describes an apparatus in which the minimum and maximum widths of a pulse-width modulated signal can be independently adjusted in accordance with predetermined data signals. Arai U.S. Pat. No. 4,999,718 describes an apparatus with interference suppression, in which a pattern selection signal is used in accordance with a halftone area or line image on a document to inhibit the output of a non-selected pattern so that a noise component due to mutual interference is eliminated. A pulse-width modulated signal which is in linear relation with the image signal is produced.
Image quality problems continue to exist with pulse-width modulation systems, including the generation of artifacts giving fuzzy characters and lines, as well as xerographic stability problems which are a result of inconsistent toner output distribution from one output image to the next. FIG. 4 and graphs A, B, C and D therein, provide an example of a pulse-width modulation scheme. Graph A shows a series of pixel clock pulses providing input pixel width. Graph B shows a video input signal such as might be derived from an input scanning device. Graph C shows a periodic triangular pulse width modulation function. Modulating input signal B with periodic triangular signal C produces a modulated output signal shown in Graph D of FIG. 4.
Fuzzy characters or lines using pulse width modulation are caused by aliasing or high frequency overlapping of successive periods. One method of overcoming fuzzy characters using pulse width modulation functions is to use two modulation functions with different frequencies. For example, one modulation function (one pixel width) is at a higher frequency than another (two pixel width). By increasing the modulation function when image spatial frequency increases, sharp contrasts between lines and text are preserved. A decrease in frequency modulation during areas of low spatial frequency, enables varying shades of gray to be generated.
In xerographic laser printers, stability problems caused by variances in a xerographic process place less toner on pixel edges due to xerographic threshold movement. The xerographic threshold is the effective level to which a charged photoreceptor surface exposed to a laser is discharged. In other words the threshold represents the level at which a surface is considered to be black or white (or more generically, one color or another for color separations). if the threshold is unstable between two identical copies, there exists a variance in overall color (or toner) distribution.
Another method of increasing virtual device resolution is through resolution enhancement, which takes place prior to creating output image spots. Resolution enhancement only uses pattern recognition technique over an n spot.times.m spot region about a pixel being processed to look for specific problems that may be corrected by either moving the spot left or right, changing the spot size or adding spots. Image quality problems addressed by the resolution enhancement technique include reduced jaggies (or stair-stepping) on or near vertical or horizontal lines, blunted serifs (fine decorative lines added to a typeface) and toner pooling at line intersections, as shown in PCL 5 and The Laser Jet III: Hewlett-Packard Sets Standards for the 1990's by Charles LeCompte (BIS CAP International, 1990). As with other virtual device resolution improvements techniques, resolution enhancement increases perceived resolution.